目录
子查询指一个查询语句嵌套在另一个查询语句内部的查询，这个特性从MySQL 4.1开始引入。
SQL 中子查询的使用大大增强了 SELECT 查询的能力，因为很多时候查询需要从结果集中获取数据，或者需要从同一个表中先计算得出一个数据结果，然后与这个数据结果（可能是某个标量，也可能是某个集合）进行比较。
1、子查询的基本使用及分类
1.1、子查询的基本使用
-
子查询的基本语法结构：括号内为内查询（子查询），括号外为外查询（主查询）
-
在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询
-
子查询（内查询）在主查询之前一次执行完成。
-
子查询的结果被主查询（外查询）使用 。
-
注意事项
-
子查询要包含在括号内
-
将子查询放在比较条件的右侧
-
单行操作符对应单行子查询，多行操作符对应多行子查询
-
1.2、子查询的分类
1、按内查询的结果返回一条还是多条记录，将子查询分为单行子查询、多行子查询。
2、按内查询是否被执行多次，将子查询划分为相关(或关联)子查询和不相关(或非关联)子查询。
????????子查询从数据表中查询了数据结果，如果这个数据结果只执行一次，然后这个数据结果作为主查询的条件进行执行，那么这样的子查询叫做不 ????????相关子查询。
????????同样，如果子查询需要执行多次，即采用循环的方式，先从外部查询开始，每次都传入子查询进行查询，然后再将结果反馈给外部，这种嵌套 ????????的执行方式就称为相关子查询。
2、单行子查询
2.1、单行子查询比较操作符
?2.2、代码示例
#题目：查询工资大于149号员工工资的员工的信息
SELECT employee_id, last_name, salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE employee_id = 149
);
#题目：返回job_id与141号员工相同，salary比143号员工多的员工姓名，job_id和工资
SELECT last_name, job_id, salary
FROM employees
WHERE job_id = (
SELECT job_id
FROM employees
WHERE employee_id = 141
)
AND salary > (
SELECT salary
FROM employees
WHERE employee_id = 143
);
#题目：返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary = (
SELECT MIN(salary)
FROM employees
);
#题目：查询与141号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id。
#方式1:
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id = (
SELECT manager_id
FROM employees
WHERE employee_id = 141
)
AND department_id = (
SELECT department_id
FROM employees
WHERE employee_id = 141
)
AND employee_id <> 141;
#方式2：了解
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id) = (
SELECT manager_id,department_id
FROM employees
WHERE employee_id = 141
)
AND employee_id <> 141;
2.3、HAVING中的子查询
-
首先执行子查询。
-
向主查询中的HAVING 子句返回结果
#题目：查询最低工资大于110号部门最低工资的部门id和其最低工资
SELECT department_id, MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
SELECT MIN(salary)
FROM employees
WHERE department_id = 110
);2.4、CASE中的子查询
SELECT employee_id,last_name,CASE department_id WHEN(SELECT department_id
FROM departments
WHERE location_id = 1800) THEN 'Canada'
ELSE 'USA' END "location"
FROM employees
2.5、子查询中的空值问题
子查询不返回任何行，主查询也不返回任何行
SELECT last_name, job_id
FROM employees
WHERE job_id =
#子查询中空值
(SELECT job_id
FROM employees
WHERE last_name = 'Haas');
3、多行子查询
-
也称为集合比较子查询
-
内查询返回多行
-
使用多行比较操作符
3.1、多行比较操作符
3.2、代码示例?
1、IN
SELECT employee_id, last_name
FROM employees
WHERE salary IN
(SELECT MIN(salary)
FROM employees
GROUP BY department_id); 2、ANY
#题目：返回其它job_id中比job_id为‘IT_PROG’部门任一工资低的员工的员工号、姓名、job_id 以及salary
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
);3、ALL
#题目：查询平均工资最低的部门id
#MySQL中聚合函数是不能嵌套使用的。
#方式1：
SELECT department_id
FROM employees
GROUP BY `department_id`
HAVING AVG(salary) = (
SELECT MIN(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
);
#方式2：
SELECT department_id
FROM employees
GROUP BY `department_id`
HAVING AVG(salary) <= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
4、相关子查询
4.1、相关子查询执行过程
如果子查询的执行依赖于外部查询，通常情况下都是因为子查询中的表用到了外部的表，并进行了条件关联，因此每执行一次外部查询，子查询都要重新计算一次，这样的子查询就称之为关联子查询。
相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询。
说明：子查询中使用主查询中的列 ?
4.2、代码示例
#题目：查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
#方式1：使用相关子查询
SELECT last_name, salary, department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE department_id = e1.department_id
);
#方式2：在FROM中声明子查询
SELECT e1.last_name, e1.salary, e1.department_id
FROM employees e1, (
SELECT department_id, AVG(salary) avg_sal
FROM employees e2
GROUP BY `department_id`
) e2
WHERE e1.department_id = e2.department_id
AND e1.salary > avg_sal
#题目：查询员工的id,salary,按照department_name 排序
SELECT employee_id, salary
FROM `employees` e
ORDER BY (
SELECT `department_name`
FROM `departments` d
WHERE e.department_id = d.department_id
);#题目：若employees表中employee_id与job_history表中employee_id相同的数目不小于2，
#输出这些相同id的员工的employee_id,last_name和其job_id
SELECT e.employee_id, e.last_name, e.job_id
FROM employees e
WHERE 2 <= (
SELECT COUNT(*)
FROM `job_history` j
WHERE j.employee_id = e.employee_id
);4.3、EXISTS 与 NOT EXISTS关键字
-
关联子查询通常也会和 EXISTS操作符一起来使用，用来检查在子查询中是否存在满足条件的行。
-
如果在子查询中不存在满足条件的行：
-
条件返回 FALSE
-
继续在子查询中查找
-
-
如果在子查询中存在满足条件的行：
-
不在子查询中继续查找
-
条件返回 TRUE
-
-
NOT EXISTS关键字表示如果不存在某种条件，则返回TRUE，否则返回FALSE。
1、EXISTS
#题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
#方式1：自连接
SELECT DISTINCT m.employee_id, m.last_name, m.job_id, m.department_id
FROM employees e JOIN employees m
ON e.manager_id = m.employee_id;
#方式2：子查询
SELECT e.employee_id, e.last_name, e.job_id, e.department_id
FROM employees e
WHERE e.`employee_id` IN(
SELECT DISTINCT m.manager_id
FROM employees m
);
#方式3：使用EXISTS
SELECT e.employee_id, e.last_name, e.job_id, e.department_id
FROM employees e
WHERE EXISTS (
SELECT *
FROM employees m
WHERE e.`employee_id` = m.manager_id
);
2、NOT EXISTS
#题目：查询departments表中，不存在于employees表中的部门的department_id和department_name
#方式1
SELECT d.department_id, d.department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;
#方式2
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE e.department_id = d.department_id
)4.4、相关更新
使用相关子查询依据一个表中的数据更新另一个表的数据。
#题目：在employees中增加一个department_name字段，数据为员工对应的部门名称
# 第一步
ALTER TABLE employees
ADD(department_name VARCHAR2(14));
# 第二步
UPDATE employees e
SET department_name = (SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id);4.5、相关删除
使用相关子查询依据一个表中的数据删除另一个表的数据。
#删除表employees中，其与emp_history表皆有的数据
DELETE FROM employees e
WHERE employee_id IN
(SELECT employee_id
FROM emp_history
WHERE employee_id = e.employee_id);5、关于自连接和子查询效率问题
题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。
可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化。
6、练习
#1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name, salary
FROM employees
WHERE department_id (
SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'
);
#2.查询工资比公司平均工资高的员工的员工号，姓名和工资。
SELECT employee_id, last_name, salary
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
#3.选择工资大于所有JOB_ID = 'SA_MAN'的员工的工资的员工的last_name, job_id, salary
SELECT last_name , job_id, salary
FROM employees
WHERE salary > ALL(
SELECT salary
FROM employees
WHERE job_id = 'SA_MAN'
);
#4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT employee_id, last_name
FROM employees
WHERE department_id IN(
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE '%u%'
);
#5.查询在部门的location_id为1700的部门工作的员工的员工号
SELECT employee_id
FROM employees
WHERE department_id IN(
SELECT department_id
FROM `departments`
WHERE `location_id` = 1700
);
#6.查询管理者是King的员工姓名和工资
#方法1
SELECT last_name, salary
FROM employees
WHERE manager_id IN(
SELECT employee_id
FROM employees
WHERE last_name = 'King'
);
#7.查询工资最低的员工信息: last_name, salary
SELECT last_name, salary
FROM employees
WHERE salary = (
SELECT MIN(salary)
FROM employees
);
#8.查询平均工资最低的部门信息
#方式1
SELECT *
FROM departments
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
)
);
#方式2
SELECT *
FROM departments
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
);
#方式3:LIMIT
SELECT *
FROM departments
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
)
)
#方式4
SELECT d.*
FROM departments d, (
SELECT department_id, AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) t_dept_avg_sal
WHERE d.department_id = t_dept_avg_sal.department_id;
#9.查询平均工资最低的部门信息和该部门的平均工资（相关子查询）
#方式1：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) = (
SELECT MIN(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
)
);
#方式2：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) <= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
);
#方式3： LIMIT
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) =(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 1
)
);
#方式4：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) t_dept_avg_sal
WHERE d.`department_id` = t_dept_avg_sal.department_id
#10.查询平均工资最高的 job 信息
#方式1：
SELECT *
FROM jobs
WHERE job_id = (
SELECT job_id
FROM employees
GROUP BY job_id
HAVING AVG(salary) = (
SELECT MAX(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY job_id
) t_job_avg_sal
)
);
#方式2：
SELECT *
FROM jobs
WHERE job_id = (
SELECT job_id
FROM employees
GROUP BY job_id
HAVING AVG(salary) >= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY job_id
)
);
#方式3：
SELECT *
FROM jobs
WHERE job_id = (
SELECT job_id
FROM employees
GROUP BY job_id
HAVING AVG(salary) =(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 0,1
)
);
#方式4：
SELECT j.*
FROM jobs j,(
SELECT job_id,AVG(salary) avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 0,1
) t_job_avg_sal
WHERE j.job_id = t_job_avg_sal.job_id
#11.查询平均工资高于公司平均工资的部门有哪些?
SELECT department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary) > (
SELECT AVG(salary)
FROM employees
);
#12.查询出公司中所有 manager 的详细信息
#方式1：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;
#方式2：子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
);
#方式3：使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e1.`employee_id` = e2.`manager_id`
);
#13.各个部门中 最高工资中最低的那个部门的 最低工资是多少?
#方式1：
SELECT MIN(salary)
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) = (
SELECT MIN(max_sal)
FROM (
SELECT MAX(salary) max_sal
FROM employees
GROUP BY department_id
) t_dept_max_sal
)
);
SELECT *
FROM employees
WHERE department_id = 10;
#方式2：
SELECT MIN(salary)
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) <= ALL (
SELECT MAX(salary)
FROM employees
GROUP BY department_id
)
);
#方式3：
SELECT MIN(salary)
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) = (
SELECT MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal ASC
LIMIT 0,1
)
);
#方式4：
SELECT MIN(salary)
FROM employees e,(
SELECT department_id,MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal ASC
LIMIT 0,1
) t_dept_max_sal
WHERE e.department_id = t_dept_max_sal.department_id
#14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary
#方式1：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN ANY (
SELECT DISTINCT manager_id
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MAX(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
)
)
);
#方式2：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id = ANY (
SELECT DISTINCT manager_id
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) >= ALL (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
)
)
);
#方式3：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal DESC
LIMIT 0,1
) t_dept_avg_sal
WHERE e.`department_id` = t_dept_avg_sal.department_id
);
#15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号
#方式1：
SELECT department_id
FROM departments
WHERE department_id NOT IN (
SELECT DISTINCT department_id
FROM employees
WHERE job_id = 'ST_CLERK'
);
#方式2：
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.`department_id` = e.`department_id`
AND e.`job_id` = 'ST_CLERK'
);
#16. 选择所有没有管理者的员工的last_name
SELECT last_name
FROM employees e
WHERE NOT EXISTS (
SELECT employee_id
FROM employees m
WHERE e.manager_id = m.employee_id
);
#17．查询员工号、姓名、雇用时间、工资，其中员工的管理者为 'De Haan'
#方式1：
SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id IN (
SELECT employee_id
FROM employees
WHERE last_name = 'De Haan'
);
#方式2：
SELECT employee_id,last_name,hire_date,salary
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e1.`manager_id` = e2.`employee_id`
AND e2.last_name = 'De Haan'
);
#18.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资（相关子查询）
#方式1：使用相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE department_id = e1.`department_id`
);
#方式2：在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal
#19.查询每个部门下的部门人数大于 5 的部门名称（相关子查询）
#方式1
SELECT department_name
FROM departments d, (
SELECT department_id, COUNT(*) num
FROM employees
GROUP BY department_id
) dept_num
WHERE d.department_id = dept_num.department_id
AND num > 5
#方式2
SELECT department_name
FROM departments d
WHERE 5 < (
SELECT COUNT(*)
FROM employees e
WHERE d.department_id = e.`department_id`
);
#20.查询每个国家下的部门个数大于 2 的国家编号（相关子查询）
SELECT country_id
FROM locations l
WHERE 2 < (
SELECT COUNT(*)
FROM departments d
WHERE l.`location_id` = d.`location_id`
);